Doroshenko, V. V. “On Semigroups of Order-Preserving Transformations of Countable Linearly Ordered Sets”.
Ukrains’kyi Matematychnyi Zhurnal 61, no. 6 (June 25, 2009): 723–732. Accessed April 11, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/3055.